1. Field of the Invention
The present invention relates to an exhaust emission control system of an internal combustion engine for cleaning exhaust gases discharged from the internal combustion engine by adsorbing unburned constituents included in the exhaust gases with an adsorbent material.
2. Description of the Related Art
An exhaust emission control system disclosed in Japanese Patent Unexamined Application Hei. 9-324621 is known as a conventional exhaust emission control system of this type. This exhaust emission control system has an adsorbing device for adsorbing hydrocarbons which are unburned constituents. This adsorbing device is disposed downstream of a three-way catalyst in an exhaust pipe, and provided in the adsorbing device are two exhaust passages which branch from each other in the vicinity of an entrance and join together in the vicinity of an exit of the adsorbing device. An adsorbent material is provided in one of the exhaust passages (hereinafter, referred to as an xe2x80x9cadsorbent material side passagexe2x80x9d) which is adapted to adsorb hydrocarbons and to release the hydrocarbons so adsorbed thereto as temperature increases. Additionally, a switchable recirculation pipe for recirculating part of exhaust gas to an induction side of an engine is connected to the adsorbent material side passage downstream of the adsorbent material. Furthermore, a switching valve is provided where the adsorbent material side passage and the other exhaust passage (hereinafter, referred to as xe2x80x9cthe other passagexe2x80x9d) join together which is adapted to open one of the exhaust passages while closing the other thereof. This switching valve is driven to open the adsorbent material side passage when a vacuum is provided which is generated when the engine is in operation.
The switching valve of the adsorbing device keeps the other passage open before the engine is started. The switching valve is controlled to open the adsorbent material side passage once the engine is started, whereby exhaust gases which have not been cleaned by the three-way catalyst which has not yet been activated are allowed to flow into the adsorbent material side passage, and hydrocarbons included in the exhaust gases are adsorbed by the adsorbent material. The exhaust gases so cleaned by adsorbing the hydrocarbons to the adsorbent material are then discharged to the outside via an exhaust pipe connected to the exit of the adsorbing device.
Thereafter, when the three-way catalyst is activated, the switching valve then opens the other passage. As this occurs, the recirculation pipe connected to the adsorbent material side passage is also opened, whereby most of the exhaust gases which have been cleaned by the three-way catalyst are allowed to flow through the other passage for discharge to the outside, whereas part of the exhaust gases flows through the adsorbent material side passage. Then, when the adsorbent material is heated by the exhaust gases which flow through the adsorbent material side passage the hydrocarbons which are now adsorbed to the adsorbent material are then released from the adsorbent material for recirculation to the induction side of the engine via the recirculation pipe. Thereafter, when the hydrocarbons have been completely released from the adsorbent material the recirculation pipe is closed, whereby exhaust gases are then allowed to flow through only the other passage to be discharged to the outside.
Additionally, in this exhaust emission control system, the switching valve is controlled as follows in order to remove from the adsorbent material deposits such as soot which are deposited to the adsorbent material while hydrocarbons are adsorbed and/or released. Namely, the switching valve is controlled to open the adsorbent material side passage when a fuel cut is executed while the engine is in operation or when the engine is in deceleration with the air-fuel ratio of exhaust gas being in lean conditions even while fuel is being supplied, whereby exhaust gases containing therein much oxygen are allowed to flow into the adsorbent material side passage so as to promote the combustion of deposits deposited in the adsorbent material to thereby remove the deposits from the adsorbent material. Thus, the recovery of the adsorbing performance of the adsorbent material and improvement of the durability thereof are attempted to be attained by removing the deposits from the adsorbent material.
As has been described above, in the conventional exhaust emission control system, the special control is required that the switching valve is driven when the predetermined conditions are established such as the fuel cut while the engine is in operation in order to remove the deposits from the adsorbent material. Moreover, since the driving of the switching valve occurs frequently while the engine is in operation, the durability of the switching vale and hence the exhaust emission control system becomes deteriorated. In addition, since the switching valve is driven by a vacuum generated by the operation of the engine, it is not possible to drive the switching valve which closes the adsorbent material side passage before the engine is started. To cope with this, in order for exhaust gases initially generated when the engine is started to be allowed to flow into the adsorbent material side passage, the switching valve needs to be controlled to be operated by estimating timing at which the switching valve is driven from the layout of the adsorbing device.
It is an object of the invention to provide an exhaust emission control system which can ensure that deposits in an adsorbent material are removed and also can improve the durability thereof without implementing any special control for switching a flow path of exhaust gases.
According to an aspect of the invention, there is provided an exhaust emission control system for an internal combustion engine provided along an exhaust system 2 having a main exhaust passage 13 connected to an internal combustion engine 1 and a bypass exhaust passage 14 which branches off and joins back to the main exhaust passage 13 for cleaning exhaust gases discharged from the internal combustion engine, the exhaust emission control system comprising 5 a switching device (an exhaust passage switching device 9 in an embodiment (hereinafter, referred to as the same when discussions are made as to the aspect of the invention)) for switching an exhaust gas flow path to either of the main exhaust passage and the bypass exhaust passage, an adsorbent material of zeolite (an HC adsorbent material 16) disposed within the main exhaust passage for adsorbing unburned constituents present in exhaust gases introduced into the main exhaust passage and releasing the unburned constituents so adsorbed as temperature increases, and a control device (an ECU 21) for controlling the switching device such that the exhaust gas flow path is switched to the main exhaust passage when the unburned constituents in the exhaust gases are allowed to be adsorbed by the adsorbent material, that the exhaust gas flow path is switched to the bypass exhaust passage when the unburned constituents so adsorbed by the adsorbent material are allowed to be released from said adsorbent material, and that the exhaust gas flow path is switched to the main exhaust passage when the release of the unburned constituents from the adsorbent material is completed.
According to the construction, the switching device is controlled such that the exhaust gas flow path is switched to the main exhaust passage when the unburned constituents in the exhaust gases are allowed to be adsorbed by the adsorbent material disposed within the main exhaust passage, that the exhaust gas flow path is switched to the bypass exhaust passage when the unburned constituents so adsorbed by the adsorbent material are allowed to be released from the adsorbent material, and that the exhaust gas flow path is switched to the main exhaust passage when the release of the unburned constituents from the adsorbent material is completed. Thus, when a catalyst normally provided along the exhaust system is still in inactivation, as in a case, for example, where the internal combustion engine has just been started, the exhaust gas flow path is switched to the main exhaust passage so that the exhaust gases containing the unburned constituents which have not yet been cleaned by the catalyst are allowed to flow into the main exhaust passage, whereby the unburned constituents in the exhaust gases are allowed to be adsorbed by the adsorbent material. Thereafter, when the catalyst is so activated that the unburned constituents in the exhaust gases can be cleaned, the exhaust gas flow path is then switched to the bypass exhaust passage so that the exhaust gases which have been cleaned by the catalyst are allowed to flow into the bypass exhaust passage and the unburned constituents adsorbed to the adsorbent material are allowed to be released from the adsorbent material. Then, the exhaust gas flow path is switched to the main exhaust passage after the release of the unburned constituents from the adsorbent material has been completed.
Thus, since the exhaust gas flow path is switched to the main exhaust passage all the time except while the unburned constituents are allowed to be released from the adsorbent material, i.e., almost all the time while the internal combustion engine is in operation, so that the exhaust gases are allowed to flow through the main exhaust passage, even if deposits such as soot are deposited in the adsorbent material when the exhaust gases pass through the adsorbent material, the adsorbent material is put in a highly heated state by the highly heated exhaust gases which flows through the main exhaust passage, and the deposits are burned with oxygen left unused in the exhaust gases due to fuel cuts taking place while the internal combustion engine is in operation, whereby the deposits can be removed from the adsorbent material. In addition, deposits comprising organic substances such as oil constituents deposited in the HC adsorbent material can be removed from the adsorbent material by heating the oil constituents themselves to a high temperature so that they are evaporated. Thus, the removal of the deposits can be ensured without any special control for switching the exhaust gas flow path, and as a result, it can be ensured that the clogging of the adsorbent material due to the deposits is prevented. In addition, the frequency at which the switching device performs the switching operation can remarkably be reduced by allowing the switching device to switch the exhaust gas flow path only when the unburned constituents are allowed to be released from the adsorbent material. As a result, the durability of the system can be improved. Furthermore, since the adsorbent material comprises zeolite which has superior thermal resistance, even if the exhaust gases flow through the main exhaust passage almost all the time when the internal combustion engine is in operation, there is no risk that the adsorbing performance of the adsorbent material is deteriorated.
In this case, preferably the bypass exhaust passage has an annular passage portion adapted to surround in an annular fashion a portion of the main exhaust passage where the adsorbent material is disposed.
According to the construction, when the exhaust gas flow path is switched to the bypass exhaust passage when the unburned constituents are allowed to be released from the adsorbent material, the exhaust gases are allowed to flow into the annular passage portion of the bypass exhaust passage. Since this annular passage portion surrounds the portion of the main exhaust passage where the adsorbent material is disposed, the adsorbent material can be heated by the exhaust gases flowing through the annular passage portion, whereby the adsorbent material can be heated quickly and easily to a temperature high enough to allow the unburned constituents to be released from the adsorbent material.
In addition, in this case, preferably the switching device has a switching valve element 15a adapted to freely move between an open position where the main exhaust passage is opened whereas the bypass exhaust passage is closed and a closed position where the main exhaust passage is closed whereas the bypass exhaust passage is opened, a biasing device (a torsion coil spring) for biasing the switching valve element to the open position, and an actuator 19 for driving the switching valve element from the open position to the closed position against the biasing device.
According to the construction, since the switching valve element adapted to freely move between the open position and the closed position is biased to the open position by the biasing device, the switching valve element only has to be driven from the open position to the closed position against the biasing device by the actuator. Namely, since the actuator does not have to be activated all the time except while the unburned constituents are allowed to be released from the absorbing material, i.e., almost all the time when the internal combustion engine is in operation, the amount of power to be consumed to operate the actuator can be reduced to a minimum level, and the reliability with which the exhaust gas flow path is switched over can be maintained high. In addition, even if the actuator is adapted to operate in conjunction with the operation of the internal combustion engine, since the exhaust gas flow path is adapted to be switched to the main exhaust passage at all times, as is different from the conventional example, there is no need to allow for timing at which the switching valve element is driven when the internal combustion engine is started.
In this case, the switching device further has a rotational shaft 15c adapted to be driven to rotate by the actuator, and an arm 15b connected between the switching valve element and the rotational shaft for driving the switching valve element in conjunction with the rotation of the rotational shaft, wherein preferably the rotational shaft and the arm are disposed in the bypass exhaust passage.
According to the construction, since the rotational shaft and the arm of the switching valve are disposed on the bypass exhaust passage side where there exists little exhaust gas flowing while the internal combustion engine is in operation, the deterioration of the rotational shaft and the arm through exposure to the exhaust gases can be suppressed, whereby the durability of the rotational shaft and the arm can be improved.